DE102005032807A1 - Combined etching and doping media for silicon dioxide layers and underlying silicon - Google Patents

Abstract

The present invention relates, on the one hand, to HF / fluoride-free etching and doping media which are suitable both for etching silicon dioxide layers and for doping silicon layers underneath. On the other hand, the present invention also relates to a method in which these media are used.

Description

The The present invention relates on the one hand HF / fluoride-free etching and Doping media, both for etching of silicon dioxide layers as well as doping underneath Silicon layers are suitable. On the other hand, the present invention relates Invention also a method in which these media are used.

State of Technology and object of the invention

By definition refers the term "solar cell" in the following mono- and multi-crystalline silicon solar cells, independent of Solar cell types based on other materials.

The Operation of a solar cell is based on the photoelectric effect, i.e. on the conversion of photon energy into electrical energy.

For silicon solar cells already doped silicon base material (usually p-doped Si) is doped with the opposite unlike charge carriers (eg phosphorus doping leads to the n ^{+ line} ), ie, a pn junction is generated.

at Entry of photon energy into the solar cell (solar radiation) become at this p-n junction charge carrier resulting in a broadening of the space charge zone and the Voltage increase leads. The Voltage is tapped by contacting the solar cell and the solar power is derived to the consumer.

• 1. Texturierung der Vorderseite der p-dotierten Si-Scheiben
• 2. n⁺-Dotierung (meist mit Phosphor)
• 3. Ätzung des PSG (Phosphorsilikatglas)
• 4. Passivierungs-/Antireflexbeschichtung mit Siliziumoxid- oder Siliziumnitridschichten

The typical process sequence for the production of solar cells is simplified in:

• 1. Texturing the front of the p-doped Si slices
• 2. n ⁺ doping (mostly with phosphorus)
• 3. Etching the PSG (Phosphorosilicate Glass)
• 4. passivation / antireflection coating with silicon oxide or silicon nitride layers

5. metallization from front and back

to Further consideration of the invention is a more detailed illustration of the step 4 necessary.

In some older production processes, the passivation layer is realized by forming a thermally generated SiO ₂ layer [1]. The mode of operation is that the unsaturated bonds on the Si surface are largely saturated and rendered ineffective there. This reduces the impurity density and recombination rate. By adjusting the SiO ₂ layer thickness to about 100 nm (= λ / 4 rule), a reflection-reducing surface is additionally produced.

From the literature, the coating with TiO ₂ layers is known, which show only a small passivation effect, but due to the higher refractive index can significantly contribute to the reduction of reflection of a solar cell [2].

For the production highly efficient solar cells has become in practice in particular the use of silicon nitride layers as a passivation layer proved to be particularly advantageous. Known are the outstanding Passivation properties from semiconductor technology, e.g. when Barrier, passivation layers in integrated circuits, FET, capacitors, etc.

To In today's technology, the coating of a mono- or multi-crystalline solar cell with silicon nitride the best Process for surface passivation and reflection reduction. It will be in newer production lines meanwhile used in mass production.

In laboratory experiments, it has proved advantageous for the increase in efficiency to dope the regions under the contacts of the emitter side higher than the surrounding n ^{+ region,} ie to carry out an n ⁺⁺ diffusion with phosphorus. These structures are referred to as selective or two-stage emitters [7]. The doping in the n ^{+ region} is on the order of 10 ¹⁸ cm ^-3 and in the n ⁺⁺ region in the range of about 10 ²⁰ cm ^-3 . Efficiencies of up to 24% are achieved on a laboratory scale with such highly efficient solar cells.

In order to obtain these differently doped areas for the selective emitter are in the Several methods are described in the literature, all based on a structuring step. Here, in most cases by application of photolithographic methods, in SiO ₂ layers - which prevent doping of the underlying silicon in the vapor phase - targeted openings are applied, which allow a local doping. This doping is usually carried out in the gas phase with POCl ₃ or PH ₃ . The doping windows are usually etched into the silica with hydrofluoric acid or buffered hydrofluoric acid.

by virtue of the very complex and expensive process control are these procedures no over the laboratory stage has been realized.

Another method is based on etching the n ^{+ +} regions with a hydrofluoric acid / nitric acid etching mixture, with simultaneous masking of the later contact areas. Also, this method has not been able to prevail in practice because of its complex procedure.

Alles This method has in common that with hydrofluoric acid or salts of hydrofluoric acid Passivation layer locally opened must become. In addition, after a rinsing and drying step, a Doping done in the gas phase.

Clearly superior to these methods is the local opening of the SiO ₂ or silicon nitride layer with the aid of an etching paste, as in US Pat DE 10101926.2 or PCT / EP 01-03317, and the subsequent doping in the gas phase with, for example, POCl ₃ .

In DE 101 50 040 A1 describes a combined etching and doping medium which is capable of etching silicon nitride layers and of doping silicon below the silicon nitride at the etched-out locations in a subsequent optional step.

In the known in semiconductor technology so-called LOCOS process in which silicon nitride is selectively etched in the presence of silicon dioxide with hot phosphoric acid at temperatures in the range of 150-180 ° C, the etching rates for silicon dioxide are well below 1 nm / min. By W. van Gelder and VE Hauser, the facts are illustrated quite clearly in the form of a diagram ["The etching of Silicon Nitrides in Phosphoric Acid with Silicon Dioxide as a Mask", J. Electrochem., S., 114 (8), 869 (1967 )]

The Evaluation of this graph shows that extrapolation of the in the diagram drawn lines showing the etching rates of silicon nitride and silicon dioxide, at a temperature of about 270 ° C, the etching rate of Silicon dioxide (intersection of the extrapolated straight line) equal that should be of silicon nitride.

In practical use, none has been sufficient even at a temperature of 300 ° C achieved high etch rate of silicon dioxide layers through which the use of based on phosphoric acid or on their salts etching media for example for photovoltaic applications makes sense. In contrast, with the same etching medium at 300 ° C by means of PE-CVD method produced silicon nitride layers of 70 nm thickness can be completely etched in less than 60 seconds.

It is therefore an object of the present invention to provide a suitable etching medium with the aid of which silicon dioxide layers of solar cells can be selectively etched with high etching rates. The object of the present invention is therefore also to provide a method for the selective etching of silicon dioxide layers for the production of selective emitter structures in solar cells, which, in addition to the etching, also permits targeted phosphorus doping for the generation of n ⁺⁺ regions.

description the invention

The solution the task of the invention is done by a method of etching of passivation and antireflective layers of silicon dioxide Solar cells according to claims 1 and 2 and its particular embodiment according to the claims 3 to 7. The implementation of this Process takes place by means of a phosphoric acid or salts containing etching medium, which in a process step over the entire surface or selectively on the too corrosive surface areas is applied and effective under heat becomes. The subject of the present invention are thus also the treated by this method silicon surfaces and the produced Solar cells.

The solution the task of the invention takes place in particular by means of an etching medium, wherein the various Forms of phosphoric acid or suitable phosphoric acid salts or compounds which decomposes on heating to the corresponding phosphoric acid be used as etchants and optionally serve as a doping component.

Full Description of the invention

The present invention is a method for etching of passivation and antireflection layers of silicon dioxide on solar cells by applying a phosphoric acid or their salts containing etching medium in a process step over the entire surface or selectively applied to the surface areas to be etched. In the method according to the invention, the silicon substrate provided with etching medium is heated over the entire surface or locally to temperatures in the range from 350 to 400 ° C. for 30 to 120 seconds and, if appropriate, for additional n ⁺⁺ doping for 1 to 60 minutes at temperatures> Heated to 800 ° C. It has been found that heating to temperatures in the range of 800 to 1050 ° C leads to good doping results. Printable, paste-like etching media are particularly suitable for carrying out the process according to the invention. Depending on the consistency, the etching medium used can be applied by spraying, spin coating, dipping or by printing by screen, stencil, stamp, pad or ink jet printing. The subsequent heating can be done on a hot plate, in the convection oven, by IR radiation, UV radiation, or microwave. For local heating, a laser, in particular for heating to temperatures> 800 ° C, an IR laser can be used. According to the invention, the claimed process can be used to produce solar cells with two-stage emitters.

object In particular, the present invention is an etching medium for etching inorganic passivation and antireflection layers on solar cells, which the various forms of phosphoric acid or suitable salts of phosphoric acid or compounds which decompose on heating to the corresponding phosphoric acids be contained, and in the process as etching or Dotierkomponenten serve.

When active components can this ortho-, meta-, pyro-phosphoric acid, and / or meta-phosphorus pentoxide or mixtures thereof, which are used both as etching and also act as doping component. It may be the etching medium used also an etching paste in which one or several ammonium salt (s) of phosphoric acid and / or mono- or di-esters of a phosphoric acid are included by thermal energy input the corrosive acting phosphoric acid release. Furthermore, in the etching medium according to the invention in paste form at least one etching and doping component, solvent, thickener and optionally additives such as defoamers, thixotropic agents, leveling agents, Deaerator and Adhesive containing.

Correspondingly composed etching medium are particularly suitable for n ⁺⁺ doping of silicon and can be used for the production of solar cells with two-stage emitters and can be advantageous used in the above-characterized method. Accordingly, solar cells produced in such a method using an etching medium of the present invention are also an object of the present invention.

While phosphoric acids and / or their salts have not proven to be suitable etching components for silicon dioxide layers under the etching conditions usually used, it has now surprisingly been found by experiments that ortho-phosphoric acid, meta-phosphoric acid, pyro-phosphoric acid or their salts and especially here the ammonium salts selected from the group (NH ₄ ) ₂ HPO ₄ and NH ₄ H ₂ PO ₄ , as well as other compounds which, on their thermal decomposition forms one of these compounds, are capable of silicon nitride at temperatures above 350 ° C. Etch layers of 120 nm layer thickness with a sufficiently high etching rate and completely remove.

By selective application, e.g. Screen printing, inkjet process or other procedures, and whole-area In addition, heating of the coated silicon substrate may be effected by a local etching. The same can be achieved by complete coating, e.g. by spin coating, spray coating or other methods, and subsequent local Heating, such as by heating with an IR laser.

The classical wet-chemical method for the etching of silicon dioxide is based on aqueous hydrofluoric acid or aqueous buffered hydrofluoric acid, for example NH ₄ HF ₂ or KHF ₂ solutions. The materials have a high toxicity. In addition, the volatile and also quite toxic gas SiF ₄ is formed during the etching of silicon dioxide.

Phosphoric acid or its salts, especially ammonium dihydrogen phosphate NH ₄ H ₂ PO ₄ and di-ammonium hydrogen phosphate (NH ₄ ) ₂ HPO ₄ , however, are toxicologically and ecologically largely harmless. Therefore, a particular advantage of this etching process is that without the use of toxic and expensive to be disposed of hydrofluoric acid or hydrofluoric acid salts, the silicon dioxide can be etched. In addition, the start and end of the etching can be easily controlled by the timing and duration of the thermal stimulation. A particular advantage is that with a selectively printed phosphoric acid (ortho-, meta- or pyro-), or one of its salts or compounds the same eg released by thermal excitation and in a second, immediately following process step, an n ⁺⁺ Doping can be generated, as is necessary in the case of the selective emitter. This doping process is known to the person skilled in the art and can be carried out, for example, as described in [7].

object The invention therefore is selective emitter structures by the Use of combined HF / fluoride-free, printable etching and Produce doping media.

As etching media be HF / fluoride-free, easy to handle mineral acids or their salts and / or mixtures thereof used in solution or in pasty Form can be present.

If the etching media are to be applied over the entire surface, they can be applied in a manner well known to the person skilled in the art, such as, for example, spin-coating, spraying, dipping. In a second step, the phosphoric acid for etching the silicon dioxide layer is excited by local heating, for example with the aid of a laser. In a further step, the phosphoric acid can now be locally heated to temperatures in the range> 800 ° C, preferably to temperatures of about 900 ° C, with a second stronger laser, as this is necessary for n ⁺⁺ doping. Particularly advantageous in this case is the use of an IR laser whose wavelength is not absorbed by the underlying silicon to avoid crystal defects. By skillful process control, the etching and doping process can now be carried out in one step according to the invention. Advantageously, the surrounding, not etched away, silicon dioxide acts as a diffusion barrier during the doping process and prevents the doping of the non-etched regions.

However, the selective coating of the silicon substrates with a pasty, phosphoric acid-containing mixture appears more favorable than the full-area coating. This can be done by a specialist known printing method such as by screen, stencil, stamp, pad printing. After the application, the substrates are heated in a subsequent step to initiate the silicon dioxide etching. This can be done on a so-called hotplate (hot plate) or by IR radiation or by another method known to those skilled in the art for heating substrates (microwave, convection oven). For etching, a temperature range of 350 to 400 ° C is advantageous. The etching time for a 120 nm thick Siliziumdioxidid layer is at 350 ° C for about 60 seconds. The etching step can be followed immediately in a next step, the heating of the substrate for 1 to 40 minutes at temperatures> 800 ° C, as they are necessary for thermal n ⁺⁺ doping with phosphorus. Over the duration and temperature, the diffusion profile of the phosphorus are controlled in the silicon in the manner known in the art.

Here can by a clever process control the etching and doping in one Step proceed directly after each other.

The Selective application is not only cheaper in terms of Material consumption, but also the throughput of the corrosive Substrates can be made much faster than they can be by serial Describe a surface possible with a laser is.

in the Contrary to the o.g. Printing process is the job by means of inkjet printing - a non-contact method - as a to name another variant. It can directly print a heated substrate and etched. Even under these conditions is through a clever litigation a simultaneous etching and doping possible.

Become as etching media Pastes used these both over the entire surface and selectively applied to the sites to be doped.

There in the pastes the caustic at the same time serves as a doping element, it can as described above the actual etching step also serve as a doping source.

To The solar cells are exposed to temperatures of 800 - 1050 ° C, wherein in the Paste contained doping element in the contacting areas diffused into and doped. All other paste components are volatile at these temperatures and burn without residue.

The Paste can thus in a single process step to the desired Areas of the to be etched surface be applied. One for the transfer the paste on the surface Particularly suitable technology with a high degree of automation is the Printing technology. Especially the screen, stencil, tampon, stamp Printing techniques are known to those skilled in this process.

From particular advantage is for Application of the etching and doping media according to the invention, that all Masking and lithography steps, usually for one application of wet-chemical etching processes or for selective doping in the gas phase are necessary, as well omitted as rinses.

• a. Ätz- und ggf. Dotierkomponente
• b. Lösungsmittel
• c. Verdickungsmittel
• d. gegebenenfalls Additive wie Entschäumer, Thixotropiermittel, Verlaufsmittel, Entlüfter, Haftvermittler

The etching pastes according to the invention are composed of:

• a. Etching and optionally doping component
• b. solvent
• c. thickener
• d. optionally additives such as defoamers, thixotropic agents, leveling agents, deaerators, adhesion promoters

The caustic effect of the proposed etch pastes relies on an acidic component that is effective by temperature excitation. This component comes from the group of phosphoric acid (ortho, meta, pyro) and their salts, preferably ammonium salts, selected from the group (NH ₄ ) ₂ HPO ₄ and NH ₄ H ₂ PO ₄ .

The etching component is in a concentration range of 1 - 80 wt% based on the Total mass of the etching paste in front. Due to the concentration of the etching component, the etching and Removal rate of silica are significantly influenced.

In Experiments showed that by the addition of a strong oxidizing Component, e.g. nitric acid or nitrates, the etch rate the phosphoric acid further increased can be. Optionally, the etching media according to the invention therefore the etching media such a component may be added, provided it is for the desired etching process conducive is.

Of the Proportion of the solvent may be in the range of 20-80% by weight based on the total mass of the etching paste lie. Suitable solvents can pure inorganic or organic solvents or mixtures the same as water, simple and / or polyhydric alcohols, Ethers, in particular ethylene glycol monobutyl ether, triethylene glycol monomethyl ether, [2,2-butoxy- (ethoxy)] - ethyl acetate.

The proportion of the thickener, the purpose of adjusting the viscosity range and reason In addition to the printability of the etchant, ie, to form a printable paste is required, is in the range of 1 - 20% by weight based on the total mass of the etching paste.

• • Cellulose/Cellulosederivate wie Ethyl-, Hydroxylpropyl-, Hydroxylethyl-, Natrium-Carboxymethylcellulose
• • Stärke/Stärkderivate wie Natriumcarboxymethylstärke (vivastar^®), Anionisches Heteropoly-Saccharide)
• • Acrylate (Borchigel^®)
• • Polymere wie Polyvinylalkohole (Mowiol^®), Polyvinylpyrolidone (PVP)
• • Hochdisperse Kieselsäuren wie Aerosil^®

The intrinsic viscosity of the etch pastes described is achieved by network-forming, swelling in the liquid phase acting thickener and can be varied depending on the desired application. Thickening agents may be organic or inorganic products or mixtures thereof:

• Cellulose / cellulose derivatives such as ethyl, hydroxylpropyl, hydroxylethyl, sodium carboxymethylcellulose
• • starch / starch derivatives such as sodium carboxymethyl starch (viva star ^®), Anionic heteropoly saccharides)
• • Acrylates (Borchigel ^® )
• • polymers such as polyvinyl alcohols (Mowiol ^®), polyvinylpyrrolidones (PVP)
• • Highly dispersed silicas such as Aerosil ^®

in the With regard to the use of cellulose / cellulose derivatives, but also the rest Thickener, it is found that only such derivatives can be used are opposite the substrate surface have sufficient adhesion, while spreading the etching medium Prevent and accurately print the thinnest lines and textures enable. For example, it has been found that xanthan derivatives are not for the purpose of this invention can be used.

inorganic Thickening agents, e.g. fumed silica, remain in contrast to the organic thickeners also in the subsequent doping step at temperatures> 800 ° C on the substrate and can be used to adjust the doping glass properties. Both types of thickeners, organic and inorganic, can also be combined with each other in the etching media, so that different compositions are chosen depending on the application can.

additives with for the wished Advantageous properties are defoamers, thixotropic agents, leveling agents / anti-caking agents, ventilator and adhesion agents. these can the printability the etching paste influence positively.

to Achieving high efficiencies in a solar cell is important that all starting materials for producing the etching paste have sufficient purity feature. In particular, the easily diffusing elements such as e.g. Copper, Iron, sodium, which should significantly shorten the carrier lifetime in silicon in concentrations <200 ppb be present.

According to the invention, these new etching and doping pastes are used in the solar cell industry for Production of photovoltaic devices such as solar cells or photodiodes. In particular, you can the pastes according to the invention be used in a two-step process for the preparation of emitter structures.

To the better understanding and for the sake of clarity, examples are given below which are within the scope of the present invention, but are not suitable, the invention to these examples restrict.

Example 1:

manufacturing and composition of the paste

In 100 g ortho-phosphoric acid 85% (Merck Art. 1.00573) was added 6 g of Aerosil 200 (Degussa-Huels AG). The resulting paste was stirred for a further 20 minutes with a paddle stirrer.

Example 2:

manufacturing and composition of the paste

In a mixture of 48.5% by weight of H ₃ PO ₄ (85% pure) and 48.5% by weight of 1-methyl-2-pyrrolidone, 3% by weight of PVP K90 were stirred in with stirring. The resulting paste was stirred for a further 20 minutes with a paddle stirrer.

An etching paste prepared in the manner described is printed with a polyester screen type 120 T on a commercial screen printing machine. On the sieve that is in 1 displayed layout formed and transferred to the substrate. As a substrate, a multicrystalline solar cell of 100 × 100 mm ² size with a full-area silicon dioxide passivation layer is used. Immediately after printing, the substrate is heated for 100 seconds at 300 ° C on a hotplate. The complete throughput of the silicon dioxide layer can be visually recognized after approx. 60 seconds. Thereafter, the substrate is placed at 850 ° C for 30 minutes in a diffusion furnace with atmospheric air.

After removal of the phosphorus glass layer, the local, high doping with phosphorus in the range of about 10 ²⁰ cm ^-3 can be detected by measuring the electrical conductivity by means of 4-point sample.

Claims (14)

Method for etching passivation and antireflection layers of silicon dioxide on solar cells, characterized in that an etching medium containing phosphoric acid or its salts is applied in one process step over the entire surface or selectively to the surface areas to be etched. A method according to claim 1, characterized in that the silicon substrate provided with etching medium is heated over the entire surface or locally to temperatures in the range of 350 to 400 ° C for 30 to 120 seconds and optionally then for additional n ⁺⁺ doping for 1 to 60 minutes to temperatures> 800 ° C, in particular to temperatures in the range of 800 to 1050 ° C, is heated. Process according to claims 1 - 2, characterized characterized in that a printable, pasty etching medium is used. Process according to claims 1 - 2, characterized characterized in that the etching medium depending on consistency by spraying, Spin-coating, dipping or by screen, stencil, Stamp, tampon or inkjet printing is applied. Process according to claims 1-4, characterized characterized in that the heating on a hot plate, in the convection oven, by IR radiation, UV radiation, or microwave. Process according to claims 1-4, characterized characterized in that for local heating a laser, in particular for the Heating to temperatures> 800 ° C, an IR laser is used. Process according to claims 1-6 to Production of solar cells with two-stage emitters. etching medium for etching of inorganic passivation and antireflection coatings on solar cells, containing as active component ortho-, meta-, pyro-phosphoric acid, and / or meta-phosphorus pentoxide or mixtures thereof, both as etchants and ethers also acts as doping component. etching medium according to claim 8, containing one or more ammonium salt (s) of phosphoric acid and / or mono- or di-esters of a phosphoric acid caused by thermal Energy input the corrosive acting phosphoric acid release. etching medium according to claims 8 - 9 in paste form, containing at least one etching and doping component, solvent, Thickeners and optionally additives such as defoamers, Thixotopiermittel, Leveling agent, deaerator and adhesion agents. Use of an etching medium according to claims 8-10 for n ⁺⁺ doping of the silicon. Use of an etching medium according to claims 8-10 for Production of a solar cell with two-stage emitters. Use of an etching medium according to claims 8-10 in a method according to claims 1-7. Solar cell manufactured using a method according to one or more of claims 1-7.